(1) Field of the Invention
The invention relates to the field of rotorcraft, and it relates more particularly to ways of electrically powering a turboshaft engine of a rotorcraft while it is being started.
In this field, several documents have been consulted, including those discussed below.
(2) Description of Related Art
In addition to Document U.S. Pat. No. 8,752,392, discussed in greater detail below, the following documents have also been taken into consideration: US 2002/063479, FR 2 858 484, US 2004/080165, and U.S. Pat. No. 6,233,935.
Document US 2002/063479 describes a turbogenerator having a compressor configured to compress a fuel oxidizer, and a combustion chamber connected to an exhaust of the compressor and configured to receive both the fuel and the oxidizer. A carburetor is configured to control the size of fuel droplets supplied to the combustion chamber in order to prevent flameout of the turbogenerator. A turbine is connected to the exhaust and configured to convert heat from the combustion gas into rotational energy. A motor/generator is configured to convert the rotational energy into electrical energy. A common shaft connects together the turbine, the compressor, and said motor/generator. A power controller is configured to control: a delivery pressure to the carburetor; a first fuel injection mechanism having a variable orifice; a second fuel injection system configured with separate injectors; a fuel heater mechanism; a cooler mechanism configured to cool the fuel; and an electric field inside the combustion chamber.
Document FR 2 858 484 describes an electrical power supply system for a vehicle that includes a battery. A step-up circuit raises the voltage delivered by the battery to a high voltage and delivers electrical energy at high voltage to an electrical power supply connection. A capacitor has a first terminal connected to the battery and a second terminal connected to the output terminal of the step-up circuit in order to store a fraction of the energy delivered by the step-up circuit so as to increase an electrical potential difference between the terminals up to a value that is equal to a difference between the high voltage and the source voltage. A control device controls the step-up circuit so that it delivers electrical energy to the power supply connection and to the capacitor at the high voltage matching a desired voltage higher than the source voltage.
Document US 2004/080165 describes a control arrangement for a turbogenerator/motor of a hybrid vehicle. For connection to an alternating current (AC) network, the arrangement has an AC generator, a turbine functionally connected to the generator, a first converter functionally connected to the generator, a second converter operationally connected between the first converter and an electricity distribution interface; and a direct current (DC) bus functionally connected to the first and second converters. Energy is delivered to the bus via the second converter in a first mode of operation of the turbogenerator, and power is delivered to the bus by the first converter in a second mode of operation of the turbogenerator. The voltage of the bus is controlled by the first converter in the second mode. The AC generator forms part of a permanent magnet generator or an induction generator. A proportional integral speed controller provides for a sampling time that is longer than the sampling time of a proportional integral power controller.
Document U.S. Pat. No. 6,233,935 describes controlling the starting of an internal combustion engine for an automotive vehicle. The starter is coupled to the engine crank shaft and a turbocharger that is provided with a turbine, the control makes provision for rotating the crank shaft with the starter in order to move the pistons that are coupled to the crank shaft, to use the air that is moved by those pistons for turning the turbine, and to cause the engine to start when the turbocharger reaches a predetermined speed. The speed of the turbocharger is deduced by measuring the time interval during which the pistons have been moving air. The starter is a starter/alternator.
The present invention relates more particularly to a rotorcraft having an electric circuit. The electric circuit regulates the supply of electrical power to a rotorcraft turboshaft engine while it is being started by taking electricity from various electrical energy sources powering an onboard electricity network of the rotorcraft, referred to below as the “onboard network”.
Such diversified sources of electrical energy comprise in particular at least one main source for generating DC and at least one secondary source formed by at least one electrical energy storage module made up of discharge members.
Rotorcraft are rotary wing aircraft in which the mechanical power needed to operate the rotorcraft is supplied essentially by one or more engines, in particular turboshaft engines.
It should be recalled that in general terms a turboshaft engine comprises in succession: a gas compressor; a combustion chamber for burning a fuel; and a turbine. When starting the engine, the gas generator generates air under pressure that is raised in temperature in the combustion chamber in order to drive the turbine in rotation. During sustained operation of the engine, the gas compressor is driven in rotation by the turbine.
There arises the problem of driving the engine on starting until a threshold is reached that enables the turbine to be driven in rotation sufficiently to enable the engine to operate in self-sustained manner.
For this purpose, the engine is provided with an auxiliary electric motor or starter for driving the gas turbine until reaching said rotary drive threshold of the turbine.
More precisely, the starter drives the gas compressor mechanically until an appropriate compression threshold is reached to enable fuel to be injected and the igniter circuit of the engine to be in operation. The starter is then kept in operation until a speed of rotation is reached for the turbine that is sufficient to enable the engine to sustain its own operation.
While starting the engine, the starter is powered electrically from the onboard network, which is maintained at a setpoint voltage, which by way of indication may be about twenty-eight volts (V).
The electrical energy powering the starter is supplied by at least one main electrical energy source powering the onboard network, referred to below as the “main” source. Such a main source is either an onboard source of electrical energy, in particular as constituted by at least one battery and/or an electric machine of the rotorcraft, or else an electrical energy source that is external to the rotorcraft and that is used on the ground in order to activate the starter.
In this context, it is necessary to supply sufficient electrical energy to the starter for driving the gas compressor while starting the engine against a resisting torque that is opposed by the turbine while it is being set into rotation, and to do this until the gas compressor is being driven at a speed that enables sustained operation of the engine.
Nevertheless, the supply of electrical energy from a main source comprising one or more batteries on board the rotorcraft may not suffice for driving the starter while starting the engine, particularly in the event of the battery(ies) being used at temperatures that are very low or if the battery(ies) is/are discharged.
Consequently, in order to supply electrical power for starting the engine from batteries on board the rotorcraft, it is necessary for the batteries to have a comfortable reserve of energy.
Nevertheless, fitting a rotorcraft with batteries of dimensions suitable for providing such a comfortable amount of energy presents the drawback of increasing the weight, the overall size, and also the number of batteries on board. Such provisions present the drawback of increasing the load carried by the rotorcraft, which is to be avoided.
Under such circumstances, proposals have been made to power the starter with electrical energy taken not only from said main source but also from a secondary source of electrical energy, referred to below as the “secondary” source, that is made up of a plurality of discharge members.
Such discharge members are suitable for providing the starter with a short high-power pulse of additional electrical energy in addition to the electrical energy being supplied by the main source.
Discharge members are electrical components that present the advantage of storing a large quantity of electrical energy in a small volume, and of being capable of restoring said electrical energy over a period that is short, of the order of a few seconds.
It may be recalled that the discharge members are generally conventional supercapacitors arranged to deliver electrical energy at a power intermediate between the electrical powers that can be supplied respectively by a battery and by electrolytic capacitors.
On this topic, reference may be made to Document EP 2 264 297 or U.S. Pat. No. 8,752,392 which describes ways of supplying electrical energy to the starter of a rotorcraft engine, while making use of a “main” source and a “secondary” source, itself made up of a module comprising a plurality of discharge members, referred to as a “discharge” module.
More particularly, Document U.S. Pat. No. 8,752,392 proposes using a discharge module to provide the onboard network, while starting the engine, with additional electrical energy supplementing the electrical energy supplied by the main source. A bidirectional direct current-direct current (DC-DC) converter regulates the operation of the discharge module as a function of the electrical energy needs of the starter as determined by the value of at least one parameter identifying progress in the starting of the engine.
It has been found in practice that the techniques described in Document U.S. Pat. No. 8,752,392 for supplying electrical energy to a starter of a rotorcraft engine leave room for improvement.
More particularly, it has been found useful to organize as well as possible the exchanges of electrical energy between the onboard network and the various sources of electrical energy that are used for supplying electrical energy to the starter while it is starting the engine.
In this context, the safety and the availability of the onboard network must not be affected by the techniques used for regulating the supply of electrical energy to the starter, in particular firstly as a result of delivering electrical energy from the discharge members into the onboard network, and secondly as a result of recharging the discharge members with electrical energy from the onboard network.
In addition, such regulation must be compatible with constraints associated with maintaining stable DC in the onboard network, given the way the electrical energy supplied by the onboard network is used by other rotorcraft equipment that consumes electrical energy.
Furthermore, it should not be forgotten that it is appropriate to organize such regulation while taking account of the constant desire in aviation to reduce the weight and the size of equipment on board a rotorcraft.